Camera objective of variable focal length

ABSTRACT

A varifocal forward lens group of a camera objective, with a pair of axially shiftable negative components bracketed by a pair of substantially fixed positive components, has its negative third component more widely separated from its positive fourth component in a wide-angle position of minimum overall focal length than in a telephoto position of maximum overall focal length. The effective diameter of that third component is just large enough to let a bundle of axially incident light rays fully illuminate the fourth component in the wide-angle position, with less than full illumination in the telephoto position. The positive first component is also just large enough to provide full illumination of the effective area of the third component by axial rays in the telephoto position.

FIELD OF THE INVENTION

Our present invention relates to a camera objective of the pancratic type, having a variable focal length, in which a four-component varifocal forward lens group is coaxially juxtaposed with a fixed-focus rear lens group.

BACKGROUND OF THE INVENTION

In such objectives it is customary to assemble the varifocal front group from two axially shiftable components of negative refractivity bracketed by two positively refracting components whose axial position is substantially fixed (allowance being made for a limited axial adjustability, for focusing purposes, of especially the first component or at least a front lens forming part thereof). A typical varifocal or pancratic objective of this description has been disclosed in U.S. Pat. No. 3,433,559 and in our copending application Ser. No. 713,224 filed Aug. 10, 1976.

There are certain relationships in such a system between their optical qualities, such as relative aperture and varifocal ratio, and their structural parameters including front-lens diameter and total axial length. These relationships are well understood in the art; reference may be made, by way of example, to Austrian Pat. No. 317,576.

It is generally convenient to design the varifocal front group as an afocal system in order to allow interposition of ancillary elements, such as a shutter, a diaphragm and a reflex prism, in a field of parallel rays between the two groups. Since the fourth component of the front group is positively refracting, the combination of the three preceding components must be of dispersive character so that axially incident rays diverge from the axis between the third and fourth components. The extent of this divergence increases, of course, in proportion to the axial separation of the two components; an axial ray bundle fully illuminating the stationary fourth component passes, therefore, through a smaller portion of the third component when the latter is in a forward position than when it lies close to the fourth component.

Systems are known (e.g. from Austrian Pat. Nos. 317,576 and 316,891) in which the axial movement of the third component, designed to keep constant the image plane of the objective during changes of the overall focal length, follows a substantially symmetrical course as the second component is shifted rearwardly from a wide-angle position of minimum focal length f_(min) to a telephoto position of maximum focal length f_(max). In such a case, the third component moves forwardly during a changeover from f_(min) to an intermediate focal length f_(med) and then recedes at about the same rate during a changeover from f_(med) to f_(max), being substantially equidistant from the fourth component in the two limiting positions of the varifocal range.

Alternatively, e.g. as known from French Pat. No. 1,459,076 and German published Specification No. 2,164,937, the motion of the third component may be an unsymmetrical function of the displacement of the second component, so chosen that the third component approaches the fourth component more closely in the telephoto position (f_(max)) than in the wide-angle position (f_(min)).

OBJECT OF THE INVENTION

The object of our present invention is to provide a more compact varifocal objective of the type specified in which the lens diameters, especially that of the front lens, can be significantly reduced with only a minor loss of illumination in certain positions of adjustment.

SUMMARY OF THE INVENTION

In accordance with our present invention, the first three components of a varifocal forward lens group as defined above have lens diameters sufficient for full illumination of the fourth component by axially incident light rays in the wide-angle position, in which the third and fourth components are relatively widely separated, with such dimensioning of the third component as to make its effective area sufficient for full illumination of the fourth component by axially incident light rays in the wide-angle position, yet with restriction of that bundle of light rays in the telephoto position to less than full illumination of the fourth component.

Thus, taking advantage of the aforedescribed unsymmetrical law of displacement in which the third and fourth components are far apart with the overall focal length f = f_(min) but are closely spaced with f = f_(max), we provide a maximum aperture ratio in the wide-angle position and a slightly reduced aperture ratio in the telephoto position in which a small loss of luminosity at the center of the image (e.g. on the order of one diaphragm stop) can be readily tolerated. This minor restriction of the relative aperture of the objective at the telephoto end of the varifocal range allows a considerable reduction in the diameter of the front lens which can thus be manufactured much more inexpensively and accommodated in a camera housing of correspondingly decreased dimensions. The lowering of the aperture ratio at large overall focal lengths has the incidental advantage of an increased depth of field in the telephoto position.

According to another feature of our invention, the effective area of the third component is circumscribed by an ancillary diaphragm carried thereon, preferably at the forward face of a constituent lens of that component. The ancillary diaphragm may be part of the lens mount for that component.

BRIEF DESCRIPTION OF THE DRAWING

The above and other features of our invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a schematic view of a conventional pancratic objective with a symmetrical law of displacement for the third component, shown in five different positions;

FIG. 2 is a view similar to FIG. 1 but showing, again in five different positions, a pancratic objective with an asymmetrical law of displacement to which our invention is applicable;

FIG. 3 shows the objective of FIG. 2 provided with an ancillary diaphragm reducing its entrance pupil in the telephoto position;

FIGS. 4A, 4B, and 4C are more detailed presentations of an objective of the type shown in FIGS. 2 and 3, illustrated in its wide-angle, intermediate and telephoto positions, respectively;

FIG. 5 is a view similar to FIG. 4A, showing another objective according to our invention in its wide-angle position; and

FIGS. 6A and 6B are more detailed diagrammatic views of the objective of FIG. 3 in its wide-angle and telephoto positions, respectively.

SPECIFIC DESCRIPTION

The conventional pancratic or zoom objective shown in FIG. 1, which may be designed for a camera of the Super-8 type, has a varifocal front attachment consisting of a substantially stationary positive front component 1, two axially shiftable negative components 2 and 3, and a fixed positive component 4; this attachment is coaxially juxaposed with a fixed-focus rear lens group or basic lens system 5. At Z we have shown a limiting ray of an axially incident light bundle focused on a point F₅ which is a focal point of component 5 inasmuch the group 1 - 4 is considered to be afocal. Point F₅ lies on an image plane I₅ whose position is unaffected by the correlated axial displacement of components 2 and 3. Components 1 - 5 have all been represented as thin lenses.

In graph A of FIG. 1, which illustrates the wide-angle position of minimum focal length, component 2 is at extreme left while component 3 is closely spaced from component 4. With component 2 moving progressively toward the right into the telephoto position illustrated in graph E, component 3 is shifted first to the left (graphs B and C), then begins to move back to the right (graph D) and ends up at its original small spacing from component 4 (graph E).

As further shown in graph E of FIG. 1, the limiting ray Z is refracted by component 1 toward its focal point F₁ located in an image plane I₁ of that component, is intercepted by component 2 so as to seem to originate at an image plane I₂, is further refracted by component I₃ which increases its divergence from the optical axis to make its apparent origin in an image plane I₃ coincide with the focal point F₄ of component 4, and then passes parallel to the axis to component 5 (the image plane of component 4 being thus at infinity).

The object distance of component 2, between that component and image plane I₁, has been designated a_(II) ; the corresponding image distance between that component and plane I₂ is designated b_(II). In an analogous manner we have designated by a_(III) the object distance of component 3 (measured from plane I₂) and by b_(III) the image distance of component 3 (extending to plane I₂). With the generally symmetrical law of displacement of component 3 the system satisfies the relationships

    a.sub.IIW = -b.sub.IIT

and

    b.sub.IIW = -a.sub.IIT

where the subscripts W and T indicate the wide-angle position of FIG. 1 (A) and the telephoto position of FIG. 1 (E), respectively. In the middle position of FIG. 1 (C), we have

    a.sub.IIM = -b.sub.IIM.

the magnification ratio β_(II) of the second component 2 in the wide-angle position is the reciprocal of that in the telephoto position, i.e.

    β.sub.IIW = 1/β.sub.IIT.

in the middle position of FIG. 1 (C), with f = f_(med), we find

    β.sub.IIM = -1.

for the third component 3 we have

    a.sub.IIIW = a.sub.IIIT,

    b.sub.IIIW = b.sub.IIIT,

    β.sub.iiiw = β.sub.iiit.

if the position of the limiting ray Z is determined by the diameter of component 4, then the effective area of component 3 varies inversely with its distance d_(III) from component 4. The radius of this effective area has been indicated at h_(III) in FIG. 1(A) and is the same in the wide-angle and telephoto positions. The relative aperture B_(III) of component 3 can thus be expressed by

    B.sub.III = f.sub.III /2h.sub.III

where f_(III) is the individual focal length of this component. It follows that, in the system of FIG. 1,

    b.sub.iiiw = b.sub.iiit.

in the system of FIG. 2 the separation d_(IIIW) between components 3 and 4 in the wide-angle position, graph A, substantially exceeds the corresponding spacing d_(IIIT) in the telephoto position of graph E. With zooming from the position of FIG. 2 (A) to that of FIG. 2 (E), component 3 moves forward into the position of FIG. 2 (B) and reaches its most advanced point in the intermediate position of FIG. 2 (C). In that position we again have the relationships

    a.sub.IIM = -b.sub.IIM

and

    β.sub.IIM = -1.

with further increases in the overall focal length, component 3 recedes into the position of FIG. 2 (D) before reaching its final position closest to component 4 according to FIG. 2 (B).

Instead of the equations relating to the system parameters of FIG. 1, we now have the following inequalities:

    a.sub.IIW < -b.sub.IIT,

    -b.sub.IIW > a.sub.IIT,

    d.sub.IIIW < d.sub.IIIT,

    b.sub.iiiw < b.sub.iiit.

the final inequality results from the fact that, owing to the close spacing of components 3 and 4 in the telephoto position, radius h_(IIIT) is significantly larger than the corresponding radius h_(IIIW) in the wide-angle position.

In accordance with our present invention, and as shown in FIG. 3, the effective radius h_(III) is limited in the telephoto position (graph E) to the value h_(IIIW) which it assumes in the wide-angle position (graph A). This limitation is here brought about by the provision of an ancillary diaphragm 6 fastened to the front face of component 3, the presence of this diaphragm being without effect upon the width of the axially incident light bundle in the positions of graphs B and C in which the spacing of component 3 from component 4 exceeds that of graph A. In FIG. 3(D) the axial separation of these components is slightly less than in graph A so that diaphragm 6 begins to restrict the effective diameter of component 3, thereby reducing the entrance pupil of the objective. In FIG. 3(E) this restriction results in a limiting ray Z' traversing the front component 1 at a point whose distance h'_(I) from the axis is substantially less than the corresponding distance h_(IT) of ray Z. Thus, the diameter of component 1 can be materially reduced without significantly restricting the brightness of the image except in positions at or close to the telephoto end of the varifocal range.

The ancillary diaphragm 6 may be part of the lens mounting of component 3. Even if that diaphragm were omitted, the reduction of the diameter of the front component would have an equivalent effect on the axially incident ray bundle.

The relationships briefly discussed above will be better understood from a reference to FIGS. 6A and 6B in which the focal points of positive component 1, negative components 2, 3 and positive component 4 have been indicated at F₁, F₂, F₃ and F₄. An axially incident bundle of parallel light rays, with limiting rays Z, impinges upon component 1 and converges toward its focal point F₁ whose image plane I₁ also constitutes the object plane O₂ of component 2. The light bundle intercepted by the latter component appears to originate at an image plane I₂ which is also the object plane O₃ of component 3. The relative positions of components 2 and 3 are so chosen that the image plane I₃ of component 3, also constituting the object plane O₄ of component 4, always passes through the focal point F₄ whereby the light bundle is focused on infinity after leaving the lens group 1 - 4.

In view of the afocality of this lens group, focal point F₄ constitutes the vertex of a light cone whose base has a diameter equal to that of component 4. As component 3 approaches component 4, its own effective diameter must increase if it is to pass all the rays of this light cone that can be collected by component 4 in the wide-angle position of FIG. 6A. The incident cylindrical light bundle corresponding to this cone fills only a limited central zone of front component 1. In the telephoto position of FIG. 6B, however, component 1 must have a large diameter in order to allow full illumination of component 4. By making the effective diameter of component 3 in the telephoto position equal to that in the wide-angle position, we can substantially reduce the diameter of component 1 with only a minor diminution of the total quantity of incident light as indicated by the hatched areas in FIG. 6B.

In FIGS. 4A, 4B and 4C we have shown the wide-angle position, the middle position and the telephoto position of a representative objective conforming to the one indicated diagrammatically in FIGS. 1, 2 and 3. Component 1 comprises a front lens member in the form of a doublet consisting of a negative meniscus L₁ with radii r₁, r₂ and thickness d₁ cemented onto a nearly planoconvex lens L₂ with radii r₂, r₃ and thickness d₂. Component 1 further includes two positive lenses L₃ (radii r₄, r₅ and thickness d₃) and L₄ (radii r₆, r₇ and thickness d₄) with intervening air spaces s₁ and s₂. A variable air space s₃ separates lens L₄ from the axially shiftable component 2 consisting of a singlet L₅ in the form of a negative meniscus (radii r₈, r₉ and thickness d₅) separated by an air space s₄ from a doublet consisting of lenses L₆ (radii r₁₀, r₁₁ and thickness d₆) and L₇ (radii r₁₁, r₁₂ and thickness d₇). Another variable air space s₅ lies between component 2 and component 3 which consists of a single lens L₈ in the form of a negative meniscus with radii r₁₃, r₁₄ and thickness d₈. In dotted lines we have illustrated an alternate position of component 3 designed to facilitate, in a manner known per se, the taking of pictures in a so-called macro range with image ratios between 1: -10 and 1: -1. A third variable air space s₆ lies between component 3 and axially fixed component 4 which in this instance consists of two air-spaced biconvex lenses L₉ (radii r₁₅, r₁₆ and thickness d₉) and L₁₀ (radii r₁₇, r₁₈ and thickness d₁₀), the intervening air space having been designated s₇.

A reflex prism P of thickness d_(p), whose planar faces have been designated r_(a) and r_(b), is separated by an air space s₈ from lens L₁₀ and by a diaphragm space s_(d) from component 5, the latter space containing an iris diaphragm 7 distinct from the invariable ancillary diaphragm 6 on face r₁₃ on lens L₈. Component 5 consists of five air-spaced singlets, namely a biconvex lens L₁₁ (radii r₁₉, r₂₀ and thickness d₁₁), a positive meniscus L₁₂ (radii r₂₁, r₂₂ and thickness d₁₂), a biconcave lens L₁₃ (radii r₂₃, r₂₄ and thickness d₁₃), a planoconvex lens L₁₄ (radii r₂₅, r₂₆ and thickness d₁₄) and another positive meniscus L₁₅ (radii r₂₇, r₂₈ and thickness d₁₅); the intervening air spaces have been designated s₉, s.sub. 10, s₁₁ and s₁₂.

In the following Table A we have given representative numerical values for the parameters r₁ - r₂₈, d₁ - d₁₅ and s₁ - s₁₂ of the optical system of FIGS. 4A - 4C, based upon a mean overall focal length f_(med) = 1 in the middle position of FIG. 4B. The Table also gives the refractive indices n_(d) and the Abbe number ν_(d) of the various lenses.

                                      TABLE A                                      __________________________________________________________________________             Thicknesses and                                                        Lens Radii                                                                             Separations     n.sub.d                                                                            ν.sub.d                                                                         Diameter                                       __________________________________________________________________________          r.sub.1                                                                           = +                                                                               21.871                                                              L.sub.1        d.sub.1 = 0.15                                                                          1.805                                                                              25.4                                                    r.sub.2                                                                           = +                                                                               3.172                                                               L.sub.2        d.sub.2 = 0.46                                                                          1.607                                                                              59.5                                                    r.sub.3                                                                           = -                                                                               262.556                                                                            s.sub.1 = 0.01                                                       r.sub.4                                                                           = +                                                                               3.740                                                               L.sub.3        d.sub.3 = 0.32                                                                          1.689                                                                              49.5                                                    r.sub.5                                                                           = +                                                                               30.344                                                                             s.sub.2 = 0.01                                                       r.sub.6                                                                           = +                                                                               3.047                                                               L.sub.4        d.sub.4 =0.29                                                                           1.744                                                                              44.8                                                    r.sub.7                                                                           = +                                                                               8.896                                                                              0.061    ... f.sub.min                                                         s.sub.3 =  0.951                                                                        ... f.sub.med                                                         1.773    ... f.sub.max                                               r.sub.8                                                                           = +                                                                               5.897                                                               L.sub.5        d.sub.5 = 0.08                                                                          1.691                                                                              54.7                                                    r.sub.9                                                                           = +                                                                               1.020                                                                              s.sub.4 = 0.39                                                       r.sub.10                                                                          = -                                                                               1.317                                                               L.sub.6        d.sub.6 = 0.15                                                                          1.847                                                                              23.8                                                    r.sub.11                                                                          = -                                                                               0.954                                                               L.sub.7        d.sub.7 = 0.06                                                                          1.569                                                                              63.1                                                    r.sub.12                                                                          = +                                                                               6.800                                                                              1.540    ... f.sub.min                                                         s.sub.5 =  0.575                                                                        ... f.sub.med                                                         0.184    ... f.sub.max                                               r.sub.13                                                                          = -                                                                               1.018                                                               L.sub.8        d.sub.8 = 0.06                                                                          1.744                                                                              44.8                                                                               D.sub.8eff = 0.78                                   r.sub.14                                                                          = -                                                                               3.854                                                                              0.416    ... f.sub.min                                                         s.sub.6 =  0.490                                                                        ... f.sub.med                                                         0.059    ... f.sub.max                                               r.sub.15                                                                          =+ 5.601                                                               L.sub.9        d.sub.9 = 0.17                                                                          1.620                                                                              60.3                                                                               D.sub.9 eff = 1.11                                  r.sub.16                                                                          = -                                                                               2.361                                                                              s.sub.7 = 0.01                                                       r.sub.17                                                                          ` +                                                                               3.879                                                               L.sub.10       d.sub.10 = 0.14                                                                         1.620                                                                              60.3                                                    r.sub.18                                                                          = -                                                                               5.870                                                                              s.sub.8 = 0.06                                                       r.sub.a                                                                           =  ∞                                                             P              d.sub.p = 0.32                                                                          1.517                                                                              64.2                                                    r.sub.b                                                                           =  ∞                                                                            s.sub.d = 0.21                                                       r.sub.19                                                                          = +                                                                               1.752                                                               L.sub.11       d.sub.11 = 0.18                                                                         1.620                                                                              60.3                                                    r.sub.20                                                                          =  -                                                                              6.856                                                                              s.sub.9 = 0.01                                                       r.sub.21                                                                          = +                                                                               1.015                                                               L.sub.12       d.sub.12 = 0.18                                                                         1.620                                                                              60.3                                                    r.sub.22                                                                          = +                                                                               2.066                                                                              s.sub.10 = 0.25                                                      r.sub.23                                                                          = -                                                                               1.059                                                               L.sub.13       d.sub.13 = 0.14                                                                         1.847                                                                              23.8                                                    r.sub.24                                                                          = +                                                                               1.059                                                                              s.sub.11 = 0.21                                                      r.sub.25                                                                          =  ∞                                                             L.sub.14       d.sub.14 = 0.21                                                                         1.713                                                                              53.8                                                    r.sub.26                                                                          = -                                                                               0.772                                                                              s.sub.12 = 0.01                                                      r.sub.27                                                                          = +                                                                               0.953                                                               L.sub.15       d.sub.15 = 0.14                                                                         1.620                                                                              60.3                                                    r.sub.28                                                                          = +                                                                               4.297                                                               __________________________________________________________________________

This system has a minimum focal length f_(min) = 0.46, a maximum focal length f_(max) = 3.44, a relative aperture varying between 1: 1.2 (in the wide-angle position) and 1 : 1.5 (in the telephoto position), and an image diagonal 2y' = 0.47. As indicated in the Table, the effective diameter D_(8eff) of lens L₈ has the numerical value 0.78. Thus, the objective has a varifocal ratio f_(max) : f_(min) of about 8 : 1, with f_(min) ≈ 2_(y) '. The reduced diameter of front component 1, realized by a decrease of the relative aperture in the telephoto position, amounts to about 85% of the diameter that would be required for full illumination of component 4 in that position.

FIG. 5 shows another embodiment of our invention in the wide-angle position. Its front component 1' comprises a positive doublet consisting of lenses L'₁ (radii r'₁, r'₂ and thickness d'₁) and L'₂ (radii r'₂, r'₃ and thickness d'₂) closely followed by two positive menisci L'₃ (radii r'₄, r'₅ and thickness d'₃) and L'₄ (radii r'₆, r'₇ and thickness d'₄), the intervening air spaces s'₁ and s'₂ being practically zero. A variable air space s'₃ separates component 1' from component 2' which again consists of two lens members, i.e. a singlet L'₅ (radii r'₈, r'₉ and thickness d'₅) and a doublet made up of lenses L'₆ (radii r'₁₀ , r'₁₁ and thickness d'₆) and L'₇ (radii r'₁₁, r'₁₂ and thickness d'₇). After another variable air space s'₅ there follows the third component 3', again consisting of a single negative meniscus L'₈ (radii r'₁₅, r'₁₆ and thickness d'₈), separated by a third variable air space s'₆ from the fourth component 4' which in this instance is a single biconvex lens L'₉ (radii r'₁₅, r'₁₆ and thickness d'₉). A reflex prism P', of thickness d'_(p) and with planar faces r'_(a), r'_(b), is separated from lens L'₉ by an air space s'₇ and from the rear component 5' by a diaphragm space s'_(d).

Component 5' consists here of four singlets, namely a positive lens L'₁₀ (radii r'₁₇, r'₁₈ and thickness d'₁₀), a positive lens L'₁₁ (radii r'₁₉, r'₂₀ and thickness d'₁₁), a negative lens L'₁₂ (radii r'₂₁, r'₂₂ and thickness d'₁₂) and a positive lens L'₁₃ (radii r'₂₃, r'₂₄ and thickness d'₁₃); the intervening air spaces have been designated s'₈, s'₉ and s'₁₀.

Representative numerical values for parameters r'₁ - r'₂₄, d'₁ - d'₁₃ and s'₁ - s'₁₀, again based on a mean focal length f_(med) = 1, are given together with refractive indices n_(d) and Abbe numbers ν_(d) in the following Table B.

                                      TABLE B                                      __________________________________________________________________________             Thicknesses and                                                        Lens Radii                                                                             Separations     n.sub.d                                                                            ν.sub.d                                                                         Diameter                                       __________________________________________________________________________          r'.sub.1                                                                          = +                                                                               123.127                                                             L'.sub.1       d'.sub.1 = 0.11                                                                         1.805                                                                              25.4                                                    r'.sub.2                                                                          = +                                                                               2.801                                                               L'.sub.2       d'.sub.2 = 0.43                                                                         1.620                                                                              60.3                                                    r'.sub.3                                                                          = -                                                                               5.218                                                                              s'.sub.1 = 0                                                         r'.sub.4                                                                          =+ 2.308                                                               L'.sub.3       d'.sub.3 = 0.17                                                                         1.622                                                                              53.2                                                    r'.sub.5                                                                          = +                                                                               4.175                                                                              s'.sub.2 = 0                                                         r'.sub.6                                                                          = +                                                                               1.643                                                               L'.sub.4       d'.sub.4 = 0.24                                                                         1.643                                                                              48.0                                                    r'.sub.7                                                                          = +                                                                               3.972                                                                              0.031    ... f.sub.min                                                         s'.sub.3 =  0.994                                                                       ... f.sub.med                                                         1.373    ... f.sub.max                                               r'.sub.8                                                                          = +                                                                               4.332                                                               L'.sub.5       d'.sub.5 = 0.04                                                                         1.691                                                                              54.7                                                    r'.sub.9                                                                          = +                                                                               0.473                                                                              s'.sub.4 = 0.23                                                      r'.sub.10                                                                         = -                                                                               1.128                                                               L'.sub.6       d'.sub.6 = 0.04                                                                         1.620                                                                              60.3                                                    r'.sub.11                                                                         = +                                                                               0.626                                                               L'.sub.7       d'.sub.7 = 0.15                                                                         1.805                                                                              25.4                                                    r'.sub.12                                                                         = +                                                                               3.189                                                                              1.402    ... f.sub.min                                                         s'.sub.5 =  0.258                                                                       ... f.sub.med                                                         0.155    ... f.sub.max                                               r'.sub.13                                                                         = -                                                                               0.990                                                               L'.sub.8       d'.sub.8 = 0.01                                                                         1.788                                                                              47.4                                                                               D'.sub.8eff = 0.47                                  r'.sub.14                                                                         = -                                                                               3.946                                                                              0.122    ... f.sub.min                                                         s'.sub.6 =  0.303                                                                       ... f.sub.med                                                         0.028    ... f.sub.max                                               r'.sub.15                                                                         = +                                                                               0.982                                                               L'.sub.9       d'.sub.9 = 0.10                                                                         1.682                                                                              48.2                                                                               D'.sub.9eff =0.57                                   r'.sub.16                                                                         = -                                                                               3.593                                                                              s'.sub.7 = 0.04                                                      r'.sub.a                                                                          =  ∞                                                             P'             d'.sub.p = 0.22                                                                         1.517                                                                              64.2                                                    r'.sub.b                                                                          =  ∞                                                                            s'.sub.d = 0.17                                                      r'.sub.17                                                                         = +                                                                               1.885                                                               L'.sub.10      d'.sub.10  = 0.09                                                                       1.744                                                                              44.8                                                    r'.sub.18                                                                         = -                                                                               2.897                                                                              s'.sub.8 = 0.02                                                      r'.sub.19                                                                         = +                                                                               0.653                                                               L'.sub.11      d'.sub.11 = 0.11                                                                        1.744                                                                              44.8                                                    r'.sub.20                                                                         =  ∞                                                                            s'.sub.9 = 0.03                                                      r'.sub.21                                                                         = -                                                                               0.869                                                               L'.sub.12      d'.sub.12 = 0.31                                                                        1.847                                                                              23.8                                                    r'.sub.22                                                                         = +                                                                               0.506                                                                              s'.sub.10 ` 0.05                                                     r'.sub.23                                                                         = +                                                                               1.404                                                               L'.sub.13      d'.sub.13 = 0.21                                                                        1.735                                                                              41.6                                                    r'.sub.24                                                                         = -                                                                               0.533                                                               __________________________________________________________________________

This system has a minimum focal length f_(min) = 0.27 and a maximum focal length f_(max) = 2.78. Its relative aperture ranges from 1 : 1.8 (in the wide-angle position) to 1 : 2 (in the telephoto position); its image diagonal is 2y' = 0.31. The effective diameter of lens L'₈, given in the Table, is D'_(8eff) = 0.47. The minimum focal length f_(min) falls here short of the image diagonal 2y' by more than 10%. The objective of Table B is not as fast as that of Table A but has a higher varifocal ratio, slightly exceeding 10 : 1.

In practice, the reduction of the diameter of the front component 1 cannot be carried quite as far as diagrammatically illustrated in FIG. 6B because of the adverse effect this would have on nonaxially incident light bundles. Hence, component 3 and diaphragm 6 are chosen to determine the relative aperture of the system throughout its varifocal range. 

We claim:
 1. In a camera objective of variable focal length including a varifocal forward lens group and a fixed-focus rear lens group centered on a common optical axis, said forward lens group consisting of a substantially axially fixed positive first component, an axially shiftable negative second component, an axially shiftable negative third component and an axially fixed fourth component, said second and third components being displaceable between a wide-angle position of minimum overall focal length and a telephoto position of maximum overall focal length with maintenance of an invariant image plane behind said rear lens group, said third component approaching said fourth component more closely in said telephoto position than in said wide-angle position, the combination of said first, second and third components being of dispersive character, the improvement wherein said first, second and third components have lens diameters sufficient for full illumination of said fourth component by axially incident light rays in said wide-angle position but wherein said third component has an effective area sufficient for full illumination of said fourth component by a bundle of axially incident light rays in said wide-angle position but restricting said bundle to less than full illumination of said fourth component in said telephoto position.
 2. The improvement defined in claim 1 wherein said third component is provided with diaphragm means circumscribing said effective area.
 3. The improvement defined in claim 2 wherein said diaphragm means is carried on a front face of a constituent lens of said third component.
 4. The improvement defined in claim 2 wherein said diaphragm means forms part of a lens mount for said third component.
 5. The improvement defined in claim 1 wherein said first component consists of four lenses L₁, L₂, L₃ and L₄, said second component consists of lenses L₅, L₆ and L₇, said third component consists of a lens L₈ and said fourth component consists of lenses L₉ and L₁₀, said lenses having radii of curvature r₁ - r₁₈, axial thicknesses d₁ - d₁₀ and axial spacings s₁ - s₇ whose numerical values, based upon a numerical value of unity for an intermediate overall focal length, together with their refractive indices n_(d) and Abbe numbers ν_(d) are substantially as given in the following table:

    ______________________________________                                                                n.sub.d                                                                              ν.sub.d                                        ______________________________________                                         r.sub.1 = +                                                                             21.9                                                                                  d.sub.1 = 0.15     1.81  25                                    r.sub.2 = +                                                                             3.2                                                                                   d.sub.2 = 0.46     1.61  60                                    r.sub.3 = -                                                                             262.6                                                                                 s.sub.1 = 0.01                                                 r.sub.4 = +                                                                             3.7                                                                                   d.sub.3 = 0.32     1.69  50                                    r.sub.5 = +                                                                             30.3                                                                                  s.sub.2 = 0.01                                                 r.sub.6 = +                                                                             3.0                                                                                   d.sub.4 = 0.29     1.74  45                                    r.sub.7 = +                                                                             8.9                                                                                   0.06        ... f.sub.min                                                      s.sub.3 =  0.95                                                                            ... f.sub.med                                                      1.77        ... f.sub.max                                      r.sub.8 = +                                                                             5.9                                                                                   d.sub.5 = 0.08     1.69  55                                    r.sub.9 = +                                                                             1.0                                                                                   s.sub.4 = 0.39                                                 r.sub.10 =  -                                                                           1.3                                                                                   d.sub.6 = 0.15     1.85  24                                    r.sub.11 = -                                                                            1.0                                                                                   d.sub.7 = 0.06     1.57  63                                    r.sub.12 = +                                                                            6.8                                                                                   1.54        ... f.sub.min                                                      s.sub.5 =  0.58                                                                            ... f.sub.med                                                      0.18        ... f.sub.max                                      r.sub.13 = -                                                                            1.0                                                                                   d.sub.8 = 0.06     1.74  45                                    r.sub.14 = -                                                                            3.9                                                                                   0.42        ... f.sub.min                                                      s.sub.6 =  0.49                                                                            ... f.sub.md                                                       0.06        ... f.sub.max                                      r.sub.15 = +                                                                            5.6                                                                                   d.sub.9 = 0.17     1.62  60                                    r.sub.16 = -                                                                            2.4                                                                                   s.sub.7 = 0.01                                                 r.sub.17 = +                                                                            3.9                                                                                   d.sub.10 = 0.14    1.62  60                                    r.sub.18 = -                                                                            5.9                                                                   ______________________________________                                    


6. The improvement defined in claim 5 wherein the effective diameter of lens L₈ has a numerical value of substantially 0.78.
 7. The improvement defined in claim 1 wherein said first component consists of four lenses L'₁, L'₂, L'₃ and L'₄, said second component consists of lenses L'₅, L'₆ and L'₇, said third component consists of a lens L'₈ and said fourth component consists of a lens L'₉, said lenses having radii of curvature r'₁ - r'₁₆, axial thicknesses d'₁ - d'₉ and axial spacings s'₁ - s'₆ whose numerical values, based upon a numerical value of unity for an intermediate overall focal length, together with their refractive indices n_(d) and Abbe numbers ν_(d) are substantially as given in the following table:

    ______________________________________                                                                n.sub.d                                                                              ν.sub.d                                        ______________________________________                                         r'.sub.1 = +                                                                            123.1                                                                                 d'.sub.1 = 0.11    1.81  25                                    r'.sub.2 = +                                                                            2.8                                                                                   d'.sub.2 = 0.43    1.62  60                                    r'.sub.3 = -                                                                            5.2                                                                                   s'.sub.1 = 0                                                   r'.sub.4 = +                                                                            2.3                                                                                   d'.sub.3 = 0.17    1.62  53                                    r'.sub.5 = +                                                                            4.2                                                                                   s'.sub.2 = 0                                                   r'.sub.6 = +                                                                            1.6                                                                                   d'.sub.4 = 0.24    1.64  48                                    r'.sub.7 = +                                                                            4.0                                                                                   0.03        ... f.sub.min                                                      s'.sub.3 =  0.99                                                                           ... f.sub.med                                                      1.37        ... f.sub.max                                      r'.sub.8 = +                                                                            4.3                                                                                   d'.sub.5 = 0.04    1.69  55                                    r'.sub.9 = +                                                                            0.5                                                                                   s'.sub.4 = 0.23                                                r'.sub.10 = -                                                                           1.1                                                                                   d'.sub.6 = 0.04    1.62  60                                    r'.sub.11 = +                                                                           0.6                                                                                   d'.sub.7 = 0.15    1.81  25                                    r'.sub.12 = +                                                                           3.2                                                                                   1.40        ... f.sub.min                                                      s'.sub.5 =  0.26                                                                           ... f.sub.med                                                      0.16        ... f.sub. max                                     r'.sub.13 = -                                                                           1.0                                                                                   d'.sub.8 = 0.01    1.79  47                                    r'.sub.14 = -                                                                           4.0                                                                                   (0.12       ... f.sub.min                                                      s'.sub.6 =  0.30                                                                           ... f.sub.med                                                      0.03        ... f.sub.max                                      r'.sub.15 = +                                                                           1.0                                                                                   d'.sub.9 = 0.10    1.68  48                                    r'.sub.16 = -                                                                           3.6                                                                   ______________________________________                                    


8. The improvement defined in claim 7 wherein the effective diameter of lens L'₈ has a numerical value of substantially 0.47. 